Condenser board for Namco System 12 games

[Esom]

I'm trying to wrap my head around the requirements for condensing several Namco System 12 games similarly to RiddledTV's MK board. The kick harness on the fighters have 4 or 6 buttons plus the right speaker + and -. I assume a Schottky diode will be sufficient for isolate the buttons from one another, but what do I do with the speaker portion? Do I have to use an actual switch?

And am I getting this right?

IN_BTNn o----+----|<|----o A_BTNn   (to PCB A button input)
             |
             +----|<|----o B_BTNn   (to PCB B button input)

PCBs on the anode side, control panel on the cathode?